The present exemplary embodiments relate to traffic signals. It finds particular application in conjunction with utilizing light emitting diodes with traffic signals. One particular application for such an LED traffic signal is interfacing with control systems previously utilized with incandescent traffic signals, and it will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Traffic signals are employed to regulate motorists and pedestrians via various commands. These commands are provided by various illuminated elements with particular colors and/or shapes that are each associated with an instruction. Elements are conventionally illuminated via incandescent bulbs which use heat caused by an electrical current to emit light. When electrical current passes through a filament (e.g., tungsten), it causes the filament to heat to the point that it glows and gives off light. Such illumination can be covered with a colored lens and/or template to provide a meaningful instruction that can be viewed in a variety of external lighting conditions.
The filament is a resistive element in the incandescent bulb circuit. The amount of current drawn by the filament is proportional to its impedance. This impedance value increases as the temperature of the filament increases. Thus, a conventional lamp has a larger initial current draw which drops in proportion to the increase in the filament impedance. This variation in current draw is known and a predetermined range can be utilized to monitor the lamp operation. As such, a lamp failure condition can be identified based on the amount of current drawn by the filament. In one example, the filament fails (e.g., breaks) causing the impedance approaches an infinite value and the current value decreases to almost zero. If the current drawn is outside of the predetermined range, a responsive action can be initiated by a current monitor or other control system.
Current monitors detect the failure of traffic signal lamps by monitoring the current drawn by the lamps. A current lower than a predetermined threshold is interpreted as a lamp failure by the current monitor. LED signals draw significantly less current than traditional incandescent signals for which current monitors were originally developed. Some current monitors therefore interpret functional LED signals as having failed. LED traffic signals require a dedicated electronics circuitry to prevent current monitors from detecting current loss in installations where such monitors are used.
Unlike the incandescent-based lamps, which use a single large bulb, the LED-based lamps consist of an array of LED elements, arranged in various patterns. When viewed from a distance, the array appears as a continuous light source. LED-based lamps have numerous advantages over incandescent lamps, such as greater energy efficiency and a longer lifetime between replacements than conventional signals. Some of the longer lifetime results since a plurality of LEDs are employed, wherein a light can be utilized even if some of the LEDs in the array have failed.
What are needed are systems and methods to utilize LED signals that seamlessly interface with conventional traffic signal monitoring systems.